This invention relates to a process for preparing hydrolysis-stable, inorganic, enzyme carriers having organic enzyme binding sites and consisting of a round, macropermeable agglomerate of primary particles arranged to form a mulberry-type substructure, to the enzyme carriers prepared by the process of the invention, and to the carrier catalysts prepared by immobilizing enzymes on these carriers.
It is known in the art that carrier catalysts (i.e. carriers coated with enzyme) having high enzyme density can be prepared based on porous silicon dioxide or aluminum oxide carrier materials. These carrier catalysts exhibit a controlled material exchange and high productivity when used. Both the carrier materials of silicon dioxide and those of aluminum oxide cannot be compressed and can be prepared with defined pore structure and pore distribution as well as high pore volume.
Silicon dioxide as a carrier material is indeed an inexpensive and easily accessible starting material which can easily be prepared to have the properties required for enzyme carriers (for example porosity, grain fraction, bulk density etc) and which is also resistant to abrasion when subject to the hydrodynamic loads occurring in industrial practice. However, silicon dioxide carrier materials have the disadvantage that they are not stable to hydrolysis in the neutral to weakly alkaline pH range. This negative property leads in practice to problems when enzymes are immobilized on the silicon dioxide carrier material, the optimum activity of these enzymes requiring a carrier having neutral to weakly alkaline pH value. One example of such an enzyme is glucose isomerase which is used industrially immobilized on silicon dioxide carriers for partial conversion of glucose carrier into a mixture of glucose and fructose (high-fructose content syrup, HFS). The disadvantages of the silicon dioxide carrier material are clearly shown using this example of a carrier catalyst, since the silicon dioxide coated with enzyme is also clearly subjected to hydrolysis during the reaction of the glucose substrate (isomerization). The hydrolysis of the carrier results in a marked loss of carrier mass and an associated loss of enzyme, as a result of which the operating time of isomerization reactors may be considerably reduced. In extreme cases a blockage of the enzyme bed may even occur in the reactor with the result that the total residual activity of the carrier catalyst still present in the reactor (possibly up to 40% of the initial activity) may be lost.
Carriers made from aluminum oxide are stable toward hydrolysis, but the process of preparing them is complex. The starting material must be initially calcined at temperatures which are significantly above 1,000.degree. C., then broken to obtain the required grain fraction and sieved. High losses of material occur during the breaking and sieving due to the chalky structure of the material, and the usable aluminum oxide carrier fraction obtained in this manner retains the undesirable property of chalking even after additional treatment measures and subsequent enzyme coating (carrier catalyst). This adverse property is disadvantageous for the use of such a carrier catalyst in industrial practice since abrasion, enzyme losses and reactor blockages occur under the hydrodynamic conditions which exist in the reactor.